.Advanced Radio and Radar.pdf
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air cadet publication ACP 35 communications volume 3 - advanced radio & radar
Transcript of .Advanced Radio and Radar.pdf
air cadet publicationACP 35
communications
volume 3 - advanced radio & radar
Amendment List DateAmended by Incorporated
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CONTENTS
ACP 35COMMUNICATIONS
ii
ISSUED 2000
Chapter 1................Communicating
Chapter 2................Receivers
Chapter 3................Radar
Chapter 4................Equipments
Chapter 5................Instructors Guide
Volume 1 ................Radio Communications (Now ACP 31 Section 6)
Volume 2 ................Communications Manual (Now ACP 44)
Volume 3................Advanced Radio and Radar
Volume 4 ................Satellite Communications
Volume 3
Advanced Radio and Radar
iii
COMMUNICATING
CHAPTER 1
35.3.1-1
COMMUNICATING
Exchange of Information
1. Communication may be defined as the "exchange of information" and assuch is a two-way process. Speech is one of the simplest methods of communicationthere is – and as you know, to use it effectively you need your voice to "transmit" amessage (in the form of sound energy) and your ears to receive the reply. Noticethat for 2-way communications, each person needs both a method of transmittinginformation and a method of receiving it.
2. However, using sound does have some drawbacks:
a. Speed of travel is quite slow at 300 m/s (the speed of sound).
b. Sound will not travel through a vacuum – it needs a "medium"(normally air) to transmit the energy.
c. Sound does not travel very far, even if you have a loud voice.
d. The sound can be distorted by outside factors such as echoes,wind and other unwanted noises.
3. You can improve the way sound travels by replacing air with a solid material.The string in the example carries the sound much better than air – you can speakquietly into one can, and the person holding the other one against an ear can hearyou easily. And we all know the old Red Indian trick of putting one ear to the groundto detect the sounds of distant horsemen!
4. While sound works well over short distances, for long-range communicationsan alternative method must be used – radio. A radio communications system consists
Radio – uses a differentenergy
Speed of Sound
Air as a medium
Fig 1-1: Communicating through string
Exchange ofInformation
CHAPTER 1
35.3.1-2
of a transmitter (Tx), to send the message and a receiver (Rx) to receive the reply.The link between the Tx and Rx is this time not sound energy, but electromagnetic(em) energy, - radio waves. Just like light from the sun, radio waves can travel notonly through air, but also through a vacuum – and they travel at the same extremelyhigh speed.
5. The job of the transmitter is to convert information into ‘em’ radiation. Theinformation may be sound, TV pictures or digital codes similar to those used bycomputers. The ‘em’ radiation from the transmitter will then travel in all directions fromthe aerial. The receiver picks up this signal and converts the ‘em’ radiation back intoinformation.
6. Transmitters come in all shapes and sizes. Your television remote control isone, and so is that for the car alarm. Such devices will have a very small poweroutput of about 50 milliwatts. A television or a radio transmitter will, on the other hand,have a power rating of up to 500 kilowatts. These very high-powered equipments areneeded to make transmissions reach to all parts of the country.
Fig 1-2: From transmitter to receiverElectromagnetic – ‘em’energy
INFORMATION
Tx Rx
INFORMATION
COMMUNICATING
35.3.1-3
What is electromagnetic energy?
7. When an alternating electric current flows in a wire, both magnetic and electricfields are produced outside the wire. It is the combination of these two fields that form‘em’ waves. Some can be used for radio communications – radio waves. The frequencyof the alternating current will determine the frequency of the ‘em’ waves produced,and its power rating will govern the range of radiation. There is no theoretical limit tothe frequency of ‘em’ waves, and the expression "electromagnetic spectrum" hasbeen coined to embrace all radiations of this type, which include heat and light.
Frequency and Wavelength
8. Electromagnetic radiation travels in waves in a similar fashion to sound wavestravelling through air. The waves travel in all directions from their source rather likethe pattern produced when a stone is dropped into the water in a still pond.A typical wave is usually represented like this:
SOME DEFINITIONS
Frequency (f) the number of complete vibrations or fluctuations each second (i.e. cycles per sec).
Amplitude (a) the distance between O on the Amplitude axis and a crest.Wavelength (ë) the distance between any two identical points in a wave (literally
the length of one wave).
Velocity (v) the speed with which the waves moves is given by the formula: v = f λλλλλ
Relationship betweenfrequency, wavelengthand velocity
What is ‘em’?
Fig 1-3: A typical waveform
Time (seconds)
0 1 2
A
Frequency = 2 Hz (i.e. 2 cycles per sec)
Amplitude
λ
CHAPTER 1
Why use electromagnetic energy
9. Using ‘em’ energy to carry our communications information has manyadvantages compared with sound energy:
a. Speed of travel is extremely fast, at the speed of light, it is always 3 x 108
metres/second (sometimes expressed as “ms-1”), which is 186,000 miles/second.
b. ‘Em’ waves will travel through a vacuum and so can be used forcommunication in space.
c. ‘Em’ waves travel a long way for a given power rating.
35.3.1-4
Advantages of ‘em’
Fig 1-3: Comparative Wavelengths in common usage
COMMUNICATING
35.3.1-5
Why use such high frequencies?
10. Aerials used for transmission or reception operate best at certain wavelengths.The length of the aerial dictates the frequency at which it will transmit and receivemost readily, and aerial lengths of ë/2 for horizontal polarisation and ë/4 for verticalpolarisation are particularly efficient. As we know the velocity of the waves and, ifgiven the frequency, we can calculate the wavelength and the best aerial lengths forthat frequency. The wavelength is calculated by dividing the velocity of the wave byits frequency.
Notice – the higher the frequency, the shorter the aerial required. What does this tellus about the operating frequency of a car-mounted CB compared to a hand heldmobile phone?
Radio
11. In 1901 the Italian engineer and physicist Gulielmo Marconi was the first manto transmit and receive transatlantic radio signals. The radio waves were sent ingroups of short and long signals by switching the transmitter “OFF” and “ON” – Morsecode. Although effective, this system did depend on the operators learning Morsecode – not something everybody could do. For a system that everyone could use,some way of making the radio waves to carry more information had to be found.
λλλλλ = v
When f is the frequency, v is the velocity (3x108 metres/second) and \ is thewavelength.
Example:What horizontally polarised aerial length would suit a frequency of 200KHz?
λλλλλ = 3 x 108
200 x 103
λλλλλ = 1.5 x 103
λ λ λ λ λ = 1500 metresTherefore an aerial length of 750 metres is required for best results.
Wavelength affectsefficient aerial length
Marconi’s firstmessage in 1901
f
CHAPTER 1
35.3.1-6
12. ‘Em’ energy can be made to carry speech if we combine the low-frequencycurrents produced by speaking into a microphone, with the high-frequency currentsthat produce radio waves. This combination process is called modulation.
Modulation
13. For the transmission of sounds such as speech and music, the sound wavesare converted by a microphone into an oscillating electric current which varies at thesame frequency as the sound wave. This is called an "audio-frequency" current.
14. An electronic circuit called an oscillator produces a continuous highfrequency(radio frequency) current which has a fixed frequency chosen from the range 100KHz to 1 GHz. This fixed-frequency alternating current produces the ‘em’ "carrier"wave. The audio-frequency (AF) current and the radio-frequency (RF) current aremixed in the transmitter so that the carrier wave is MODULATED by the AF current, insuch a way as to duplicate the pattern of sound waves fed into the microphone. Acarrier wave can be modulated in one of two ways, either by amplitude modulation(AM) or by frequency modulation (FM).
Amplitude Modulation (AM)
15. The simplest form of amplitude modulation (AM) is basically the way Marconisent his first transatlantic message. The transmitter is switched alternately "ON" and"OFF" to interrupt the carrier wave. This modulates the amplitude from maximum tozero, and then back to maximum, producing pulses which represent the dots anddashes of the Morse Code.
Fig 1-5: Amplitude Modulated carrier wave
Combining high andlow frequencies –modulation
Oscillator provides RF
A M
ON
D A S H D O T
ON ONOFF OFF
MasterOscillator
Carrier
++ =
=Audio Modulated wave
Amplifier
BufferAmplifier
PowerAmplifier
AF
FO
FO
OUTPUTS
FO - AF
FO + AF
FO
Ae
COMMUNICATING
35.3.1-7
16. Whist this system is ideal for Morse, it is not good enough for speech ormusic, because sound requires many more variations (or steps) to achieve anaccurate reproduction. An improvement is to alter the amplitude of the highfrequencytone (the carrier wave) in step with the lower frequency audio tone.
Basic AM Transmitter
17. The diagram (Fig 1-7) shows the various stages of a simple AM transmitter
.
a Master Oscillator. This generates a sinusoidal voltage (the carrier) at the requiredRF frequency (FO ). Oscillators are often crystal-controlled toensure good frequency stability.
b Buffer Amplifier. This isolates the oscillator from the power amplifying stage, andprevents instability occurring.
c Power Amplifier. This is used to increase the power of the signal to the requiredlevel before radiation from the aerial (AF).
d Amplifier. This amplifies the microphone signal to the desired level foroutput.
18. The modulation takes place in the power amplifier stage. If the inputfrequencies to the modulator are FO from the oscillator and AF from the microphone,we find that the output of the power amplifier will consist of 3 frequencies:
a. The carrier (FO ).b. The carrier minus the tone frequency (speech) (FO – AF).c. The carrier plus the tone frequency (FO + AF).
Parts of the basictransmitter
Fig 1-6: Carrier wave plus AM wave form
Fig 1-7: AM transmitter block diagram
CHAPTER 1
35.3.1-8
19. For example, if the audio frequency ranged from 300 to 3000 Hz and thecarrier was 1 MHz, then the frequencies in the output would look like:
20. In the diagram you can see two sidebands to the carrier frequency, an uppersideband and a lower sideband. Some modes of operation use only one, and this iscalled single sideband (SSB) transmission. Transmitting only one sideband reducesthe size and weight of the transmitter – important factors when talking about aircraftsystems. The great drawback with the AM system is the need for such a large bandwidth(i.e. all frequencies including both sidebands, approximately 6KHz) in a limitedfrequency spread (30 KHz to 3 MHz i.e. Medium band). This means in reality that theAM system could only have 148 stations at any one time. Try tuning through an AMband radio and see how close the stations are together! Obviously, when manytransmitters are crammed into a small band and overlap each other there is a bigproblem with signals from other transmissions breaking into the one you are using –this is known as "interference". To overcome this, the use of short-range frequencymodulated systems has become popular.
Fig 1-8: Carrier and sidebands
LOWER UPPER
1MHz-3000Hz
0.997MHz 0.9997MHz 1.0003MHz 1.003MHz
1MHz-3000Hz1MHz-300Hz 1MHz-300Hz
1MHz
1MHz
COMMUNICATING
35.3.1-9
Frequency Modulation (FM)
21. With frequency modulation, the carrier wave has a constant amplitude and amuch higher frequency than AM signals. Modulation is achieved by shifting the carrierfrequency up and down slightly in step with the tone frequency. Although this shift issmall it gives better results because it is less prone to atmospheric or manmadenoise. Try listening to an AM signal as you pass by an electric pylon or enter atunnel. The AM signal is distorted or lost, but an FM signal will be largely unaffectedby the same conditions. FM is used in the range 88-108 MHz for high qualitybroadcasting; this frequency range is within a band known as the Very High Frequency(VHF) band.
Fig 1-9: Comparison between AM and FM signals
FM – high qualitybroadcasts
AM
FM
Same Signal Signals modulated Signals recovered
AM signal poor
FM signal good
CHAPTER 1
Self Assessment Questions
1. What is the speed of light ?
a. 3 x 108 ms-1
b. 3 x 106 ms-1
c. 30 x 109 ms-1
d. 30 x 101 ms-1
2. The relationship between frequency (f), wavelength (l) and velocity oflight (v) is given in the formula:
a. velocity = frequency x wavelength (v = f x l)
b. velocity = frequency + wavelength (v = f + l)
c. velocity = frequency - wavelength (v = f - l)
d. frequency = velocity - wavelength (f = v - l)
3. If the velocity of radio waves is 3 x 108, what would be the value of l fora frequency of 3 x 106 ?
a. 1000m
b. 10m
c. 100m
d. 1m
4. What does the abbreviation SSB stand for ?
a. Single Side Band
b. Single Silicone Band
c. Ship to Shore Broadcast
d. Solo Side Band
Do not mark this pagein any way! Write youranswers on a separatepiece of paper
35.3.1-10
AudioAmplifier
Demodulator Receiver
AeLoud
Speaker
RECEIVERS
RECEIVERS
1. The first element in the process of receiving a radio message is the aerial.An aerial can vary from a length of wire supported off the ground to a complex arraydesigned to select only certain frequencies, but whatever its shape, its purpose isto detect the tiny amounts of ‘em’ energy radiated from the transmitter.
How does an aerial work?
2. If an aerial in the form of a length of wire is placed into an electromagneticfield, tiny voltages are induced in it. These voltages alternate with the frequency ofthe ‘em’ radiation and are passed to the receiver circuitry for processing. The signalstrength that the aerial inputs to the receiver is very tiny the order of 5 ì(micro) volts(0.000005 volts). Therefore the receiver circuits have to be extremely sensitive. Thecircuits must also isolate the wanted signal from all the unwanted ones being received,and this is achieved by using tuned circuits. A tuned circuit simply allows a singlefrequency to pass, thus filtering out all the unwanted signals. The best known versionof a tuned circuit is the "crystal set" or "cat’s whisker" as it was called in the 1920’sand 30’s.
Fig 2-2: A basic receiver layout
Purpose of an aerial
‘Em’ waves inducevery small voltages
Fig 2-1: The Crystal Set receiver
35.3.2-1
CHAPTER 2
CHAPTER 2
Superhet Receivers
3. In those early models of receiver the problems encountered were noise(too much interference), poor amplification, limited selectivity, poor sensitivity (abilityto remain on a station) and lack of fidelity (quality of sound).
4. To overcome some of these problems, the superheterodyne (superhet)receiver was developed. Heterodyne is the term used to describe the mixing of onefrequency with a slightly different frequency to produce something called "beats".
5. If two notes of nearly equal frequency are sounded together, a periodic riseand fall in intensity (i.e. a beat) can be heard. You can sometimes hear this when atwin-engined propeller-driven aircraft flies overhead. If the pilot has not adjusted theengines to identical rpm, you hear a "wow-wow" instead of a steady note. The beatfrequency is always the numerical difference between the two frequencies. Forexample, if an audio note of 48 Hz is sounded together with one of 56 Hz then therhythmic beat of 8 Hz (56 - 48) would be heard.
The same applies to radio waves, where the beat becomes an added frequencyknown as an intermediate frequency (IF). If a radio frequency (RF) signal with a frequencyof 3,550 MHz is received and mixed with an IF of 3.551 MHz (1 KHz higher), a beatfrequency of 1 KHz would be the result. This lower radio frequency can now beprocessed more effectively by the receiver’s electronic circuits than the higher radiofrequencies. The schematic at Fig 2-4 shows the components of a typical superhetreceiver.
Fig 2-3: Beat diagram showing softer and louder tones
Heterodyne receiversuses beats
35.3.2-2
Softer
Louder
RECEIVERS
1 RF Amplifier improves sensitivity and selectivity (not used on allreceivers).
2 Mixer changes the frequency, combines incoming with theLocal Oscillator (LO) to give Intermediate Frequency(IF).
3 LO produces a constant frequency (different fromincoming).
4 IF Amplifier usually 2 or more stages. Amplifies the mixer output(gives most of gain).
5 Demodulator (detector) extracts the intelligence from the RF signal.
6 Audio Frequency Amplifier increases the signal to required levels of output devices(speaker / headphones).
FM Receivers
6. Reception on the AM bands is limited in both quality of reproduction andbandwidth availability. FM systems are less likely to be affected by "noise" and giveincreased signal performance. The FM receiver circuitry is similar to the AM systembut uses a discriminator (also called a ratio detector) in place of a demodulator. Thediscriminator is a circuit which has been designed to detect small differences infrequencies. These differences are converted to a voltage output that represents theAF component input.
CarrierInput
RecoveredSignal
Ratio DetectorReference Source
Amplifier for Output
35.3.2-3
Fig 2-5: FM signal recovery through use of a ratio detector
Fig 2-4: A superhet receiver
FM receivers usediscriminators
Ae 1 2
3
4 5 6
CHAPTER 2
Self Assessment Questions
1. What is the purpose of an aerial on a receiver?
a. To convert the electromagnetic waves (‘em’) into tiny voltages
b. To convert the electromagnetic waves (‘em’) into large voltages
c. To convert the electromagnetic waves (‘em’) into very large voltages
d. To convert the electromagnetic waves (‘em’) into a constant voltage
2. What does superheterodyne receivers make use of?
a. Bleats
b Boats
c. Beats
d. Bullets
3. What do FM receivers use to demodulate signals?
a. Distractor
b Modulator
c. Discriminator
d. Disputer
Do not mark this pagein any way! Write youranswers on aseparate piece ofpaper
35.3.2-4
R A D I A T E D P U L S E
R A D I A T E D P U L S E
RA
NG
E
R
OTATION
RADAR
RADAR
1. As World War II approached, scientists and the military were keen to find amethod of detecting aircraft outside the normal range of eyes and ears. They foundone, and at first called it Radio Detection Finding (RDF), then RAdio Detection AndRanging (RADAR). Radar works by firing powerful radio waves towards the target,and collecting the reflected energy. The radar operators can then find the position ofthe target in terms of its range (i.e. distance) and bearing from the radar installation.Radar equipments can also find another vital fact about a target aircraft – its height.The radar equipment displays the information for the operator on a screen similar tothat found in a television.
2. There are basically two different types of radar, namely primary and secondary,and we will look at each in turn. Primary radar relies solely on energy that It hasgenerated and radiated being reflected from the target - i.e. an echo, whereasSecondary radar has some co-operation from the target – the target generates itsown ‘em’ radiation.
CHAPTER 3
RDF becomes Radar
Primary and Secondaryradar
Fig 3-1: How radar uses reflected energy to detect targets
35.3.3-1
CHAPTER 3
Primary Radar
3. Primary radar systems may be found in ground, air, ship or spaceplatforms and are used in roles such as:
Surveillance (including weather)
Early warningNavigationGround mapping (from space or aircraft)Guidance controlTarget detection and trackingTerrain following/avoidanceCollision avoidance and altitude measurement
Air Traffic Control
How it Works
4. Radars operate their high-powered radio waves in 2 different modes:pulse-modulated (pulsed) and continuous wave (CW).
Frequency
5. Most radars operate in the Ultra High Frequency (UHF) or Super HighFrequency (SHF) bands. The Frequency of operation will depend on thefunction the radar is to perform, for example, a long range search radar willoperate on a relatively low frequency, while a weapons system fire control radarwill operate at a very high frequency.
Pulse-Modulated
6. A pulsed radar uses an echo principle. In other words, the transmitterfires a very brief pulse of energy and then "listens" for an echo to return. Thespeed of radio waves in free space, as we know is 3 x 108 ms-1 (186,000 milesper second). So if we measure the elapsed time between the transmission of thepulse and its reception back at the radar, we can use the formula:Distance = Speed x Time
35.3.3-2
Radar mile
Primary radar usesreflected energy
Pulsed and CW
RADAR
To calculate the distance to the target, the time taken for a pulse to travel one mileand return to the radar is known as a "Radar Mile". The following table shows thetimes for some basic units of distance:
1 Kilometre 1000 m 6.67 ms 6 ms (6 x 10-6)
1 Statute Mile 5280 ft 10.75 ms 10 ms (10 x 10-6)
1 Nautical Mile 6080 ft 12.36 ms 12 ms (12 x 10-6)
7. The pulses from a radar are transmitted at a rate which determines the rangeof the radar, called the pulse repetition frequency or PRF.
8. In practice the PRF might range from 250pps for long-range radars to 2000ppsfor short-range radars. For long-range radar, to get a satisfactory return from a pulse,a massive one million watts (megawatt) of radio frequency (RF) power is required.This high power is used only during the brief transmission of the pulse. The transmitteris then allowed to rest until the next pulse (as shown in Fig 3-2), and the receivermeanwhile is listening for an echo.
Continuous Wave Radar (CW)
9. There are two basic types of CW radar. These are called CW Dopplerand frequency-modulated CW (FMCW).
CW Doppler
10. Consider the situation where a radar equipment sends out a pulse ofradio waves and then "listens" for an echo. If the target is moving towards thetransmitter
Range ofTarget
Range inmetres or
feet
Time forreturn of
Echo
ApproximateTime for
roughcaluculations
35.3.3-3
Radar using Dopplereffect
Radar Timetable
Pulse repetitionfrequency
Fig 3-2: Peak power wave form in a PRF signal
Radar fires pulse Radar fires pulseRadar ‘listens’
CHAPTER 3
35.3.3-4
the reflected waves become bunched up (i.e. they acquire a higher frequency), dueto the target’s velocity. If the target is moving away, the waves are spread out andtheir frequency drops slightly. A similar effect can be experienced with sound waves.When a racing car approaches, you hear the pitch of the sound getting higher (soundwaves bunching up) until it passes. Then the pitch suddenly drops lower (soundwaves spreading out). This is called the Doppler effect. The radar equipment is ableto detect these small changes in frequency (or shifts) and so determine the target’svelocity with respect to the transmitter. A similar system is used by traffic police withtheir "speed gun".
11. Of course, a target rarely approaches the radar installation head-on and inthe diagram below, the target’s track (AC) is at an angle to its bearing from the radar(AB). The velocity in direction AB is less than the true target speed in direction AC.However, by comparing changes in Doppler shift at the radar receiver over a shortperiod, the target’s velocity can be calculated.
FMCW
12. In the case of FMCW, the transmitters signal is made to vary in frequency ina controlled cyclic manner with respect to time, over a fixed band. By measuring thefrequency of the returning echo it is possible to calculate the time interval elapsedsince that frequency was transmitted, and thus the target’s range.
Fig 3-3: How Doppler frequency shift is used
C- Target track
B- RadarInstallation
FMCW
Tran
smitted
Wav
e
Ret
urn
Wav
e
A- Target
RADAR
35.3.3-5
Secondary Radar
13. It is vital to know the identity of an aircraft displayed on an air traffic controller’sscreen, particularly in a military situation. A method of identifying aircraft was first usedin the second World War and called Identification Friend or Foe (IFF). It is still in usetoday. It works by fitting all friendly aircraft with a transmitter/receiver (called atransponder) which can send a reply signal to an interrogating transmitter/ receiver(an interrogator). On the ground the system is co-located with the primary radar, butdoes not require as much power because the radio waves have only to travel oneway – the aircraft replies with its own onboard transmitter. The IFF equipment isspecifically for military use, but a civilian version does exist, called SecondarySurveillance Radar (SSR). Both systems are compatible with the groundbasedinterrogators which use a transmission frequency of 1030 MHz, while aircrafttransponders use 1090 MHz.
14. The IFF/SSR systems have been developed so that specific information canbe obtained from an aircraft. The aircraft is interrogated on 1030 MHz using codedpulses or modes. Similarly, the aircraft will respond on 1090 MHz using a standardsystem of codes. There are 3 modes in use and they are:
Mode 1 Military Aircraft Identify
Mode 2 Military Mission Identify
Mode 3 A Common Military/Civilian Aircraft Identify
B Civil Identify
C Height Encoded Data
15. IFF/SSR system provide ATC authorities with a wealth of information aboutparticular aircraft – far exceeding the amount of information gained by simply using aprimary radar. The types of information available are:
Aircraft height (direct from aircraft’s altimeter)DirectionSpeedType of aircraft
SSR modes
IFF and SSR
CHAPTER 3
35.3.3-6
16. The aircraft can also send emergency information such as:
Loss of radio communications (code 7600)
Hijack (code 7500)
SOS (code 7700)
17. The main advantages of IFF/SSR over primary radar are:
a. No clutter problems (i.e. unwanted returns from rain clouds andmountains) since transmitter and receiver operate on differentfrequencies.
b. Increased range with less transmitted power, as the radio wavesonly have to travel one way.
c. More information from each target.
d. Ability to use wide bandwidth receivers.
18. SSR has become an indispensable component in Air Traffic and Air Defencesystems because an aircraft not using SSR is less easily observed, and presents apotential collision hazard.
Aerials
19. As you know, a simple aerial can consist of a piece of wire which, whentransmitting, will radiate electromagnetic waves equally in all directions. Similarly, asimple piece of wire will receive signals from all directions. This is of limited usewhen trying to determine the direction of a particular reflection. Instead, the wavesneed to be concentrated into a beam so that the radar can be made to "look" and"listen" in one specific direction at a time.
20. Focusing radio waves into a beam requires a much more complicated aerialsystem than a simple straight wire. In order to produce a beam of radiation we need
Directing the radiationbeam
Advantages of SSR
RADAR
35.3.3-7
to radiate from a shaped area, and not a single wire. In theory, this means that forlong wavelengths great areas of aerial would be needed. To overcome this problem,reflectors are used on the aerial to reflect the radio waves in one direction. Thesituation is very similar to the reflector in a torch or headlight focusing the light into anarrow beam. To detect accurate bearings of aircraft the aerial is rotated through360°, sweeping a narrow beam of radiation in a complete circle (called scanning). Allreflections can now be plotted around a circle – with the aerial at the centre. To obtainvertical information about the aircraft the aerial is moved up and down through 90° –in a type of nodding movement. From the reflections received, accurate height andrange information can be measured.
The Display
21. Obtaining a target is only part of the detecting process. The operator needsto "see" the target in visual form. For this we use a cathode ray tube (CRT) whichworks on a similar principle to a television screen. As the time interval betweenpulses is short the screen can be calibrated in miles to match the range of the pulse.
22. At Fig 3-5, the instant the pulse is transmitted a spot appears at "A". It thentravels towards "B" at a constant speed known as the "base velocity". If a target isdetected a "blip" appears; in this case "C". Because the screen is calibrated inmiles we know the distance to the target.
CRT display
Fig 3-4: Beam patterns for differing aerial types
SIDE VIEW
PLAN VIEW
RADIATIONPATTERNEMITTED
Single Wire Yagi ArrayYagi Array Parabolic Dish
A B
C
TRACE
40 Miles
30 Miles
20 Miles
CHAPTER 3
For a moving target, the blip would travel along the line "A-B". The important factorhere is that the time-base of the CRT is synchronised to the start of the transmissionof the radar pulse. Fig 3-6 shows the output from a Type "A" display. From the pipsor marks (known as intervals) the operator can estimate the range of the target.
23. To find the bearing of a target (i.e.its direction) we need to find itsazimuth (bearing measured fromNorth). By transmitting a narrow beamof radio waves and rotating it through360°, the azimuth of any target beingilluminated can be calculated. A Type"A" display shows only the range ofa target, but it is possible to displayboth range and bearing on the sameCRT by using a plan position indicator(PPI) display. The spot on the PPIdisplays starts from the centre of thescreen and produces a radial trace.This trace moves in time with therotation of the aerial. Range rings canbe added to aid the operator in rangefinding.
35.3.3-8
Plan Position Indicator
Fig 3-5 A CRT Display Fig 3-6 Type “A” display
Fig 3-7 Range Rings
24. The height of a target can becalculated by using the slant range(distance from the radar to the target).This is calculated by using the formula
Height = Slant Range x sin 22222
The target’s ground range can be calculated by the formula:
Ground Range = Slant Range x cos 22222
25. From what you have just read, to pinpoint a target by both height andbearing requires more than one aerial. However, there is now a radar systemthat combines both of these facilities into one aerial, known as the 3-D. It worksby electronically selecting the various aerial arrays and passing the informationto the PPI display.
Factors affecting radar operations
26. There are many factors that prevent efficient operation of a radarsystem, such as:
a. Noise – unwanted signals from radio, stars and theatmosphere.
b. Interference – unwanted man-made signals such as otherradar transmitters, electrical apparatus and electrical machinery. Thecorrect siting of a radar system can reduce the effects of some of theseproblems.
c. Clutter – unwanted echoes from hills, buildings, sea, clouds,hail, rain and snow. These false echoes will weaken real echoes fromtargets, but fortunately they can be reduced by electronic techniques.
d. Target characteristics – a target’s shape and composition willhave an effect on its echo. Metal, for example, is a better reflector ofradio waves
RADAR
Factors affecting radarperformance
Slant ranges andHeight
Fig 3-8: Slant triangle
Ground range
3-D radar
35.3.3-9
Slant Range
Ground Range
Heig
ht
22222
CHAPTER 3
than wood or plastic – flat surfaces are better reflectors than curves. TheUSA’s stealth fighters and bombers make use of the different reflectingcapabilities of materials and shapes to effectively "hide" from enemy radars.
Radar Installation
27. This block diagram shows the components of a typical radar installation.
Master Timing Unit (MTU) This unit produces regular, timed pulses. It controlsthe number of pulses transmitted per second andthe start of the timebase generator, and itsynchronises the system.
Transmitter (Tx) The transmitter produces high energy RF pulsesand determines the pulse duration. The range offrequencies used is in the order of 400 MHz to40GHz.
Aerial This is used to launch the RF pulses and collect thereturns for processing.
35.3.3-10
Fig 3-9: Radar block diagram
MTU TX
RXT/R SwitchCRT
Indicator
TimebaseGenerator
RADAR
Transmit/Receive (T/R) Switch
This is an electronic device that switches both thetransmitter and receiver “ON” and “OFF”. It is importantthat the receiver is disconnected with the transmitteris firing pulses (to prevent damage). On the returncycle the transmitter is disconnected while thereceiver is online to prevent reflections beingabsorbed by the transmitter.
Receiver (Rx) The receiver collects and amplifies the returningechoes and then produces the video pulses thatare applied to the display.
CRT Indicator The CRT indicator displays the target echoes to theoperator.
Timebase Generator This unit provides the reference signal for the start ofthe transmit sequence.
35.3.3-11
CHAPTER 3
Self Assessment Questions
1. What does RADAR stand for?
a. Radar Detection and Ranging
b. Radiation Aircraft Rangingc. Radio Detection and Ranging
d. Ranging and Direction Radio
2. How many modes are there in IFF/SSR?
a. 4b. 1c. 2
d. 3
3. What does SSR stand for?
a. Secondary Surveillance Radarb. Service Surveillance Radioc. Single Side Radio
d. Second Sense Radar
4. What does CRT stand for?
a. Cathode Radio Tubeb. Cathode Ray Tubec. Cathode Radiation Test
d. Capacitor Resistor Transistor
5. What is the purpose of a timebase generator in a Radar?
a. Provides the reference signal to start the transmit sequenceb. Synchronise the T/R switchc. Provide a reference signal for the receiverd. Used to launch the pulses and collect them on return
35.3.3-12
Do not mark this pagein any way! Write youranswers on aseparate piece ofpaper
EQUIPMENTS
Equipments
1. We have already looked at the general principle of operation of both radiocommunication and radar. In this Chapter we will look more closely at a variety ofdifferent types of equipment used in the RAF, to see how and where they are used.
Precision Approach Radar (PAR)
2. The purpose of PAR is to plot the approach of an aircraft wishing to land andallow ATC to give accurate guidance to the pilot to achieve a safe landing. Thesystem can be used in poor weather conditions (i.e. low cloud, limited visibility),thus reducing interruptions to a station’s flying programme.
3. PAR consists of a Radar Head cabin connected to the ATC operations cell inthe control tower. The Radar Head is mounted on a strong framework and can rotatearound a central point. This means the cabin can be turned to serve whicheverrunway is in use. The turning mechanism can be operated remotely from the operationcell in ATC, or manually in the cabin itself.
Fig 4-1: A typical layout of a PAR cabin
PAR
35.3.4-1
CHAPTER 4
Azimuth beam
Elevation beam
4. The Radar Head has 3 distinctive assemblies – the Azimuth antenna module,the Elevation module and the Radar Cabin. PAR offers the facility of allowing the safeapproach in bad weather to a point where the pilot’s “visual” acquisition of the runwayallows a safe landing. It can guide pilots to the runway from up to 15 nautical milesaway.
Principle of Operation
5. A narrow wedge shaped beam is transmitted from both PAR antennas. One isa horizontal beam (2° wide by 0.5° high) and the other a vertical beam (0.5° wide by2° high). These beams are then interlocked to give a cross shaped beam. The scanningmotion is controlled by the ATC operator in the control tower and allows the aircraft tobe "captured" in the beam pattern. This information is then displayed to the controlleron a screen with two displays. One display is of the elevation scan, the other showsthe azimuth scan. Using both of these displays the controller is able to guide theaircraft down a safe "glide path" to approach the runway on the correct course.
Instrument Landing System (ILS)
6. The ILS is a pilot-interpreted system which provides accurate guidance to therunway for a safe landing without a ground controller.
CHAPTER 4
35.3.4-2
The radar head
Cross shaped beam
Fig 4-2: Aircraft are caught in the cross beams
ILS
EQUIPMENTS
35.3.4-3
7. An ILS ground installation is situated near the runway. It transmits signals thatallow a pilot (who is on a landing approach) to accurately locate the aircraft’s positionrelative to the touchdown point. These signals provide the pilot with:
a. A visual indication (on a cockpit instrument) of the aircraft’s azimuthrelative to the runway centre line.
b. A visual indication (on the same cockpit instrument) of the aircraft’selevation in relation to the correct descent angle.
c. Both an audio (via radio headset) and visual (a flashing light on thecockpit instrument) indication of the aircraft’s distance from touch down.
Fig 4-3: A plan view of ILS
Line of Approach
Carrier 108-112mHz (VHF)
LT Localiser TransmitterGT Glidepath TransmitterMM Middle MarkerOM Outer Marker
90Hz
90Hz
RUNWAY
RUNWAY
AIRFIELD BOUNDARY
150Hz
150Hz
3500 ft
400HzMOD
1300HzMOD
75MHz
Glide Path
Carrier 329-335MHz (VHF)
3.9nm
OM
OM
MM
GT
LT
LTGTMM
CHAPTER 4
d. An audio indication to the pilot of the identity of the airfield ahead (inMorse code), to confirm that he is landing at the right airfield!
8. The ILS ground system has 3 separate elements, each providing differentinformation:
a. Localiser. The localiser gives azimuth and airfield identificationinformation (7a and 7d above), and it is installed usually some 1,000 ft beyondthe upwind end of the instrument runway.
b. Glide Path. The glide path gives elevation information (7b), and isinstalled slightly to one side of the runway, near the ideal touch-down point.
c. Marker Beacons. The marker beacons give range information, by"telling" the pilot when he is over them (7c). They are installed in a direct linewith the centre line of the runway, as follows:
(1) Outer Marker. This is located at a point where the glide slope andthe landing pattern intersect (typically 5 nm from the end of the runway).
(2) Middle Marker. This is located on line with the localiser (typically1/2 to 3/4 miles from the end of the runway).
(3) Inner Marker. This is installed in very few systems. If used itwould be positioned at the beginning of the runway.
9. To use the ILS a pilot must position the aircraft (using radar or other means)in line with the instrument runway at a range of some 20 to 25 miles. As he fliestowards the runway, first, he passes over the outer marker which "tells" him he has 5miles to go. Second, the localiser and glide-path beams will be giving indicationson the cockpit instruments, and the pilot has total ILS guidance, with which he cansafely proceed on instruments towards touchdown. Third, the middle marker warningthat there is 3/4 miles or less to the runway. Shortly after, fourth, the pilot should beable to see the runway to land visually.
Marker beacons
3 elements of ILS
Localiser
Glide path
35.3.4-4
Principles of Operation
10. The instrument that gives the pilot visual indications is a meter with twopointers. One pointer indicates in which direction to fly (left or right), to align with therunway centre line. The other pointer indicates in which direction to fly (up or down),to align with the correct glide-path. When the two pointers cross at the centre of thedisplay it indicates that the aircraft is on the glide-path and the correct heading forlanding. The instrument also has warning flags which remain "set" until there is sufficientsignal strength for the system to operate. There are also ‘dots’ on the display, to helpthe pilot in determining the flight path adjustments required to gain the correct approachangle or direction.
11. The localiser radiates two radio beams, one modulated at 90 Hz frequencyand the other modulated at 150 Hz. If the aircraft is ‘off’ course to the left, 90 Hz isdominant and the azimuth pointer on the cockpit instrument moves to the right. If theaircraft is ‘off’ course to the right of the centre line, 150 Hz is dominant. If the pilot is oncourse, the instrument shows no difference in the signal, by aligning with the centreof the dial.
Above Glide Path On Glide Path Below Glide Path
EQUIPMENTS
35.3.4-5
Fig 4-5: How the glidepath looks to the pilot
CHAPTER 4
35.3.4-6
Similarly, the glide path equipment transmits 2 radio beams modulated at 90Hz and 150 Hz and the pilot can tell whether the aircraft is too high or too low from theglide path pointer, which reacts to the strengths of the signal received (see Fig: 4-6).
12. ILS is an important tool for safe handling of aircraft during the landing stageof flight. All ILS installations must conform to International Civil Aviation Organisation(ICAO) standards. These standards are high for the best reasons – safety.
Digital Resolution Direction Finding (DRDF)
14. When used as a primary aid, this ground-based equipment provides adirection fix for aircraft, but it can also be used as a backup navigation aid, or as anauto-triangulation system.
Fig 4-6: The ILS pointers in an aircraft cockpit
ICAO standards
Fig 4-7: The ILS localiser aerial (note the photographshows a technicians training setup. There would be nooffice block at a real site)
DRDF
Signed strength low - flags set Signed strength OK - flags retracted
EQUIPMENTS
Fig 4-8: A typical DRDF equipment site
Fig 4-9: How triangulation is used to locate aircraft in distress
Vectors
Bearing
15. DRDF provides the controller with information on bearings of aircraft in thefollowing forms:
a. Digital pulses are used to give a digital read-out and a vector display.
b. Direct Current (DC) voltage proportional to the angle of the bearing.
35.3.4-7
ABERDEEN
NEWCASTLE
LIVERPOOL
Aircraft sendingDISTRESS CALL
CHAPTER 4
This is displayed on the operator’s console.
c. Digital pulses are combined with information from other installations to providean exact aircraft position on a large scale map that is situated at one of the UK’s twomain control centres (this is auto-triangulation).
Principle of Operation
16. The DRDF is used primarily for aircraft in distress, and it helps air trafficcontrollers pinpoint an aircraft accurately. The ‘distressed’ aircraft will transmit a codewhich is detected by a DRDF station and used to determine a directional bearing ofthe aircraft. This information is passed to a main control centre, which uses similarinformation from other installations to triangulate the aircraft’s position. There are twocontrol centres in the UK, one is at West Drayton and the other is at Prestwick – bothserving as ‘hubs’ to a network of outstations.
TACAN
17. A Tactical Air Navigation (TACAN) beacon operates as a transponder byproviding regular transmissions of bearing information. This information, the identityand range of the beacon, is available to all aircraft within a 200 miles range of it. Anyaircraft fitted with the correct equipment can interrogate the beacon. One TACAN cangive accurate bearing, distance and identification information to 100 (correctlyequipped) aircraft simultaneously.
Brief System Description
18. Cockpit instruments indicate the range and bearing of the beacon from theaircraft, which the aircrew use to fix the aircraft’s position. The beacon also transmitsan identification code, so the aircrew can identify which beacon is being used. ForTACAN to operate as a complete system, both ground and airborne installations arerequired. The ground installation contains a transmitter/receiver and an antenna array.When the ground base receives and decodes incoming signals from aircraft it theninitiates a response sequence. The beacon also provides an identification morsecode signal at fixed intervals.
Centres at Prestwickand West Drayton
Auto-triangulation
Distance and bearing
TACAN
35.3.4-8
N
1
2
3
E
S
W
20
60
EQUIPMENTS
Principle of Operation
19. The request for distance information is generated in the aircraft by distanceinterrogation signals (DIS). These DIS signals are randomly-generated codes whichare sent to the beacon. The beacon receives the code and immediately re-transmitsit back to the aircraft. The installation in the aircraft waits for the reply to its code – itcan calculate its distance from the beacon by the time taken between transmissionand reception. Then the information is displayed on a meter infront of the pilot. Tomeasure the compass bearing from aircraft to beacon, the TACAN transmits a 15 Hzsignal – rotated through 360°. This signal has a power peak as it passes throughEast. The aircraft equipment uses this as a reference to calculate where it is in relationto North, see Fig 4-11.
Aircraft interrogatesTACAN
Fig 4-10: TACAN provides bearing information
Aircraft 1 is 60° from East (in the negative) therefore the calculation isEast- 60° to give a bearing of 30° with respect to North.
Aircraft 2 is at East and there is no deduction or addition so it is 90° withrespect to North.
Aircraft 3 is 110° from East (in the positive) therefore the calculation is110° + 90° to give a bearing of 200° with respect to North.
35.3.4-9
CHAPTER 4
20. TACAN is a useful navigation aid for aircraft going on long sorties because itallows pilots to fix their position accurately, and helps them to remain on course.
Airfield Communications System
21. It is all very well detecting the position of an aircraft using navigation aids,but controllers need to communicate effectively with the pilot. The controllers mayalso need to talk to emergency agencies such as fire or ambulance, in the event ofan emergency. Mascot Minicomms was established as a system of communicationto give controllers access to both radio and landline communications for this reason.
22. Mascot Minicomms is the interface between many different communicationssystems available to the RAF and enables the ATC controller to "patch" togetherdifferent support agencies. For example, in the event of an emergency, a pilot usingthe aircraft’s radio, can talk directly to a doctor using a telephone in a hospital.
United Kingdom Air Defence Ground Environment (UKADGE)
23. UKADGE is a network of radars (both fixed and mobile) that together coverthe whole of the UK and its airspace. Many individual sensor units such as ships or
Patching together
Mascot Minicomms
Fig 4-11: A mobile radar provides cover where needed
UKADGE
35.3.4-10
EQUIPMENTS
35.3.4-11
airborne early-warning aircraft, input information to the system that provides a largescaleoverall picture of the UK’s airspace. The control centre then has the information neededto make a judgement of any threat and how to deal with it. UKADGE is one of theworlds most modern data processing and communication systems.
Defence Communications Network (DCN)
24. The DCN is a tri-service common network for communications. The types ofinformation carried on the DCN are; operational, meteorological and administrative.This network is worldwide and uses HF radio, long-distance cables and satellites totransmit signals from sender to receiver, wherever they may be. This system is verysimilar to the civilian telex system or fax. A person writes a message, which is thentransmitted to its destination where the recipient gets a paper copy of the message.The DCN is a modern communications system and uses computers to decide thebest route through the system for the messages to take. For security reasons themessages being carried may be encrypted, so that should they be intercepted theywould be unreadable to anyone who does not know the code.
Strike Command Integrated Communications System (STCICS)
25. The STCICS system replaced the ageing HF communication system in theearly 1970’s. At that time the system was new and had up-to-date technology toperform its task. As the years have passed, improvements have been made to keepthe equipment modern and the system in good working order. The system has twoidentical control centres which pass information to the ‘user’ units. The system providesspecific services to its ‘users’ including:
a. Scheduled broadcasts giving:
(1) Meteorological information.
(2) Airfield states (i.e. Is it either “open” or “closed”).
b. Flight watch – this is the initial radio contact, for aircraft that are enteringUK airspace.
c. Message switching and relay.
DCN
STCICS
CHAPTER 4
35.3.4-12
26. With the advent of networking technology and its availability to the RAF,the advantages in expanding the system are obvious. The system could beused more diversely, and would increase both the type and number of users.
RAF Fixed Telecommunications System (RAF FTS)
27. The RAF FTS is important because it supports UKADGE in the defenceof UK airspace. If a target is detected, a central control decides the action to betaken. Good communications to other operational units, support units andemergency services are essential. The RAF FTS ensures goodcommunications between all these agencies and authorities. The systemsprovides for 3 areas:
a. Voice – person to person (either secure or not)
b. Recorded messages – written orders or signals (achieved via DCN).
c. Data – transfer of data is important and circuits carrying data normallyhave dedicated lines.
28. The RAF FTS consists of a variety of different types of media used tocarry information (called Boxer) and various types of equipment used to sendand receive the information (called Uniter).
Boxer
29. This is the network of Service-owned lines and information links thatcarry information all round the country. It includes fibre optic, microwave andsatellite links.
Uniter
30. Uniter provides the switching and terminal equipment which allows theuser to communicate information to a receiver. Communication will includevoice, recorded messages and data as described above.
RAF FTS
Boxer – data links
Uniter – hardware
EQUIPMENTS
Satellite Communications
31. With the advent of reliable satellite communications and advances innew technology, the use of HF systems has reduced over recent years inpreference to satellite communications. Satellite communications provides highspeed data links over a much broader bandwidth. This subject area is coveredmore extensively in "Satellite Communications" ACP 35 Volume 4.
BOXER
35.3.4-13
CHAPTER 5
Self Assessment Questions
1. What does PAR stand for?
a. Precision Approach Radarb. Primary Aircraft Radarc. Portable Aircraft Radar
d. Pin-point Approach Radar
2. What information comes from an ILS Localiser?
a. Height
b. Azimuthc. Range
d. Elevation
3. What does TACAN stand for?
a. Tactical Air Communications and Networkingb. Technical Air Navigationc. Tactical Air Navigation
d. Terminal Approach Radar
4. What is the name of the system that can link an ordinary telephone to annaircraft’s radio?
a. Uniterb. RF FTSc. Boxer
d. Mascot Minicomms
5. What is the name of the air defence system used in the UK?
a. United Kingdom Air Defence Ground Environmentb. United Kingdom Defence of Air Ground Equipmentc. Air Defence Ground Environment (United Kingdom)d. Ground Equipment for United Kingdom’s Air Defence
35.3.4-14
Do not mark this pagein any way! Write youranswers on a separatepiece of paper
INSTRUCTORS GUIDE
35.3.5-1 NOTES
Bandwidths Used in Broadcasting
Radio transmitters used for broadcasting sound programmes in the low, medium and highwavelengths are normally amplitude modulated. These modulation frequencies are fromapproximately 50Hz to 5Hz. If both sidebands are broadcast then for a 5KHz signal the spaceoccupied will be 10KHz. There are places in the world that only allow the modulationfrequencies to reach a maximum of 4.5KHz, thus giving a bandwidth of 9KHz.
Frequency modulated signals for broadcast are used in VHF radio and are about 15KHz persideband. The main difference is that the sidebands may be extended up to six times thatwidth. This means that a broadcast may be in the order of 180KHz in width. This is due to thehigher frequencies with more space.
INSTRUCTORS GUIDE
CHAPTER 1
Chapter 1 Para 20
Signal Bass Channel Bandwidth Bandwidth
Morse (CW) 20 Hz 100 Hz
5 Unit teleprinter 25 Hz 120 Hz
Line telephony 3.1 KHz 4 KHz
(using single sideband)
Low quality sound 5 KHz 9 KHz
High quality sound 15 KHz 200 KHz
405 line television 3 MHz 6 MHz
625 line television 6.5 MHz 8 MHz
CHAPTER 5
INSTRUCTORS GUIDE
CHAPTER 1
Crystal Set Receiver
The diagram above shows a simple receiver. It is possible to build this receiver as aproject or to gauge the understanding of the subject but is not mandatory.The ‘em’ waves are concentrated within the ferrite rod.The inductance produced forms a resonant circuit with the variable capacitor.The rod has 50 turns and they are to be all wound in the same direction.By adjusting the capacitor the resonance can be varied so stations within the mediumwave band can be selected.Demodulation is achieved by using the diode coupled with the fixed capacitor.The headphones then convert the electrical variations into audible intelligence.At point A, a long length of wire should be attached to act as an aerial.The circuit should be earthed at point B, this will improve signal strength.
35.3.5-2 NOTES
Chapter 2 Para 2
Fig 2-1: The Crystal Set receiver
1 Ferrite Rod and 6” x 3/8” coil of 50 turns with insulated wire2 500pF Variable capacitor3 100pF Fixed capacitor4 High Impedance Headphones5 OA81 or OA91 Diode
A
B
1 2 3 4
5
INSTRUCTORS GUIDE
INSTRUCTORS GUIDE
CHAPTER 2
Noise
Noise in receivers can be a very serious problem and great care is taken to eliminate it. Mosttypes of noise have their own sound and a name to describe them by. Here are two examples:
Hum A steady note of low frequency caused by the power supply breaking through to thereceiver circuitry. This is caused by poor screening.
Hiss A steady note of high frequency and can be likened to a whistle.
Noise is used in certain equipments to simulate traffic and in so doing keep the circuits active(TACAN is an example of this type of use).
Codes
Many systems have been used to communicate over distances. Perhaps the best known ofthese is semaphore used primarily in the Navy. The first telegraphy code was Morse Code in1837. This code was a combination of dots and dashes and the original idea was for thereceiver to "read" the message using the ear. This was later modified so that incomingmessages were recorded using a device called a ticker-tape (so called because of thesound it made while punching the tape). Morse is still used today to transfer information overradio waves.
By 1874 Emile Baudot, a French inventor, had developed a new telegraphy system. Morsewas exclusive because very special training was needed to understand and work thesystem. Baudot’s system used a series of five characters (called 5 unit). This meant thatevery character had five dots (either blank or solid). The letter "A" would look like thisThis code in reality is a binary system similar to those used in computers. Five (5) unitcode is still heavily used today and forms the foundation for International TelegraphAlphabet Number 2 (ITA2), an international standard for communications.
Chapter 2 Para 3
35.3.5-3 NOTES
CHAPTER 5
INSTRUCTORS GUIDE
CHAPTER 2
Frequency - UHF and SHF
Most radars operate in the ultra high frequency (UHF) or super high frequency (SHF) bandsand the frequency of operation will depend on the function the radar is to perform.
Chapter 3 Para 5
Frequency Band Frequency Range Wavelength Range Wavelength Band
Ultra high frequency (UHF) 300 MHz - 3 GHz 1 M - 10 cm Decimetric
Super high frequency (SHF) 3 - 30 GHz 10 - 0.1 cm Centimetric
TYPICAL RADAR CHARACTERISTICS
Search Radar High peak power
Low PRF
Long pulse duration
This type is a long range radar of range 200-300 miles.
Surveillance Radar Medium peak power
Medium PRF
Medium pulse duration
This type of radar has a range of approximately 75 miles.
Secondary Surveillance Radar Low peak power
Pulses transmitted in coded groups
Transmission and reception are on different frequencies
35.3.5-4 NOTES
INSTRUCTORS GUIDE
INSTRUCTORS GUIDE
CHAPTER 3
ILS
ILS employs a battery backup system in case of mains failure so that operations can continue.The equipment is self-monitoring and is able to indicate any fault to the user (in the approachcontrol) via a remote status indicator (RSI).
The codes received by the pilot as each marker is passed are:
Outer 400 Hz keying at 2 dashes per second.
Middle 1300 Hz consisting of alternate dots and dashes.
Inner 3000 Hz keying at 6 dots per second or a station code.
TACAN
In the absence of interrogation signals, random receiver noise (squitter) is used to simulateinterrogation signals, hence maintaining the beacon’s duty cycle. The beacon is alsoresponsible for selfmonitoring.
TACAN is also available as a mobile version for emergency cover/use. This version has arange of 100 miles (half that of a fixed version).
Chapter 4 Para 6
Chapter 4 Para 17
35.3.5-5 NOTES
CHAPTER 5
INSTRUCTORS GUIDE
CHAPTER 4
SELF ASSESSMENT QUESTIONS - ANSWER SHEET
Chapter 1 Page 35.3.1-10
1a
2a
3c
4a
Chapter 2 Page 35.3.2-4
1a
2c
3c
Chapter 3 Page 35.3.3-12
1c
2d
3a
4b
5a
Chapter 4 Page 35.3.4-14
1a
2b
3c
4d
5a
35.3.5-6 NOTES
